Age and Correlation of the Yorktown (Pliocene) and Croatan (Pliocene and Pleistocene) Formations at the Lee Creek Mine

Abstract

The fossiliferous beds above the Pungo River Formation (middle Miocene) in the Lee Creek open pit mine in Beaufort County, North Carolina, are approximately 70 feet (21.3 m) thick. This thickness includes 46 feet (14 m) that is correlative with the Yorktown Formation of the type area and is referred to that unit, and, above the Yorktown, a fossiliferous section 23 feet (7 m) thick that is assigned to the Croatan Formation. The 149 species or subspecies of ostracodes identified were from 16 samples from the Yorktown and Croatan. Coefficients of faunal similarity were calculated for all samples, and the resulting matrix was subjected to unweighted pair- group cluster analysis. Three major faunal groupings were delineated. The principal faunal discontinuity occurs at the Yorktown-Croatan contact about 46 feet (14 m) above the base of the Yorktown. The beds below this level belong to the <i>Pterygocythereis inexpectata</i> and <i>Orionina vaughani </i>ostracode assemblage zones. Correlation with other Coastal Plain deposits containing planktonic foraminifers indicates that the <i>Orionina vaughani</i> assemblage zone is planktonic foraminifer zones N19 and N20 in age and that the <i>Pterygocythereis inexpectata </i>assemblage zone may approximate the lowest part of planktonic zone N19 in age. Thus, the Yorktown in the Lee Creek Mine is of early Pliocene age. This is seemingly corroborated by a K/Ar date of 4.4±0.2 my on the <i>Orionina vaughani </i>assemblage zone in Virginia.&nbsp;A third major faunal assemblage is found in the beds of the Croatan Formation, which are referable to the <i>Puriana mesacostalis</i> ostracode assemblage zone. The upper part of the Croatan can be correlated with rocks in Florida and North Carolina that have been radiometrically dated by the He/U method at about 1.8 to 1.9 mya. A tentative He/U radiometric date of 2.4 mya was obtained for the lower part of the Croatan at the mine. If a date of about 2.0 mya is used for the Pliocene-Pleistocene boundary, the Croatan as used in the mine spans the Pliocene-Pleistocene boundary.